AbstractA new data acquisition (DA) system has been developed for the MAST experiment at Culham. It has also been implemented on the COMPASS experiment as a replacement for the old PDP-11 based data acquisition system and as a test bed for MAST. The DA system is distributed across a number of computers with a DA process for each distinct diagnostic system. An object-orientated approach is taken to the control and readout of each device in the system. It is designed to be independent of the hardware interfaces used on each diagnostic. The system is flexible enough to cope with diagnostics ranging from those involving simple time evolving signals to complex spectrometers, and will incorporate a new high speed distributed timing system. This system is also being considered as the interface to the real time Plasma Control system on MAST. A distributed scheduling system is used to co-ordinate the activity of each DA process with the Central Control system for each experiment. This paper describes the architecture of this data acquisition system with particular emphasis on the core of the data acquisition system. Aspects of distributed implementation of the system on real diagnostics are discussed in a companion paper [1].